Thermoplastic synthetic polymer particles, such as polyester (or copolyester) pellets, are generally supplied to converters in a semi-crystalline form. Converters desire to process semi-crystalline pellets rather than amorphous pellets because the semi-crystalline pellets can be dried at higher temperatures without agglomerating. A typical commercial process for providing crystallized pellets involves forming the polyester polymer via melt phase polymerizing up to an It.V. ranging from about 0.5 to less than 0.70, extruding the melt into strands, quenching the strands, cutting the cooled polymer strands into solid amorphous pellets, which are then often stored for entry into a second phase known as solid state polymerization to further increase the It.V. of the pellets suitable for end use applications. The pellets are re-heated to above their Tg and then crystallized under a flow of hot nitrogen gas or hot air to prevent the pellets from sticking in the solid stating polymerization reactor, and subsequently further heated in the solid state to higher temperatures under a nitrogen purge (or vacuum) in order to continue to build molecular weight or It.V. (i.e. solid stating). Thus, crystallization is necessary to avoid agglomeration of the pellets during solid stating and during the drying step prior to extruding the melt into bottle performs.
Instead of crystallizing pellets in a stream of hot gas, we have discovered that thermally crystallizing the polyester polymer under a fluid provides numerous advantages. A continuous process for crystallizing either a polyester polymer in the form of molten droplets or spheres or in the form of solid pellets or spheres (collectively “polymer particles”) in a hot fluid such as water can be attained if the temperature of the fluid is hot enough to not only crystallize the polymer particles in a short residence time, but to crystallize the polymer particles at a high temperature sufficient to prevent the particles from sticking to each other in a dryer feeding an extrusion device. The temperature required to both crystallize and prevent the particles from sticking in these dryers is well above the boiling point of water at atmospheric pressure, and would typically range from about 130° C. to 180° C. Other fluids which boil at higher temperatures can be used, but these fluids require an extra step of washing the particles to remove any residual fluid remaining on the surface of the particle after separating the fluid from the particle. Other fluids which do not tend to stick to the particle surface can be used, but these fluids tend to have lower boiling points such that their boiling point, like that of water, is also exceeded.
Accordingly, the crystallization process is conducted under pressure to prevent the liquid from fully vaporizing, thereby keeping the particles in the liquid phase where they can be easily transported and crystallized at the temperature of the liquid. The thermal crystallization of the particles in a liquid will generate a liquid/particle mixture. At some point in the process, it is necessary to separate the particles from the fluid. This separation should be achieved without having to substantially reduce the pressure in the under-liquid crystallization zone. Several disadvantages would result by reducing the pressure either in the crystallization line or in the separation zone. One such disadvantage would be increase energy consumption to continuously pressurize the crystallization line to keep the fluid in the liquid phase. Another disadvantage is that a sudden depressurization will flash the liquid into the vapor phase, resulting in fluid loss or equipment to re-condense the vapor. In the case of water which has a high heat of vaporization, a release of pressure will flash or vaporize the water from both the surface and beneath the surface of the particle, thereby quickly cooling the temperature of the particle.
This latter disadvantage may be of no consequence, except that we have made a discovery that it would be highly desirable to maintain the temperature of the particles after separation close to the temperature of the particles before separation. Upon releasing the pressure on the particles during separation, the water on and/or in the particles will quickly vaporize, thereby reducing the particle temperature.
We have discovered that is would be desirable to separate the particles from the liquid while maintaining the system pressure, and to substantially dry the particles without also significantly reducing the particle temperature.